This invention relates to an automatic control system for a thermal power plant, and more particularly to an automatic control system of the kind described above which is effective for lessening mutual interference between individual processes and suitable for application to decentralized control of unit processes.
In order that a thermal power plant generates a desired electrical output, it is necessary to control process variables such as quantities of fuel, feed water and air, thereby generating steam at a temperature and a pressure matching the desired electrical output. However, the process variables described above are greatly interrelated with one another, and it is difficult to attain stable control of all the process variables at the same time. For example, an increase in the quantity of feed water results in a corresponding decrease in the temperature of main steam. In order to compensate for this temperature drop of main steam, the quantity of fuel must be increased, and, at the same time, air must be supplied in a quantity corresponding to the increased quantity of fuel. As described above, the process variables are closely interrelated with one another. Because of the close interrelation among the process variables, an automatic control system of very complex structure is required for the control of the thermal power plant. As a prior art example of such a control system, a system having a structure as described below is reported in a magazine entitled "Hitachi Review" Vol. 65, No. 9 (1983-9), pp. 603-608.
In the method employed in the reported system, controlling the opening of a turbine inlet control valve is controlled according to a load command signal applied to the thermal power plant. On the other hand, at the boiler side, the flow rate of feed water to the boiler is controlled according to a boiler input command signal obtained by correcting the load command signal by adding thereto a pressure compensating signal produced by subjecting a deviation of the main steam pressure from its desired value to proportional plus integral operation, and a fuel flow-rate is controlled according to a fuel command signal obtained by correcting the boiler input command signal by adding thereto a temperature compensating signal produced by subjecting a deviation of the main steam temperature from its desired value to proportional plus integral operation. Further, flow-rates of feeding gas and air are controlled by an air flow-rate command signal obtained by correcting the fuel command signal by adding thereto an oxygen concentration signal produced by subjecting a deviation of the oxygen concentration in the furnace draft gas from its desired value to proportional plus integral operation. According to the prior art method described above, main steam of good quality can be generated as a result of the control. However, the reported system is defective in that a large length of time is required until finally all of the interrelated process variables are properly corrected thereby to completely stabilize the electrical output of the plant. Also, even when the electrical output of the plant is stabilized, many terminal equipments relating to the plant control may be still unstabled, resulting in a low efficiency of the plant as a whole. Further, when any one of the compensation signal generating sections for obtaining the signals used for correcting the flow rates of feed water, fuel, gas and air on the basis of the detected pressure and temperature of main steam and concentration of oxygen in furnace gases fails to normally operate or becomes abnormal, for example, when the compensation signal generating section relating to the pressure of main steam becomes abnormal, all of feed water, fuel, gas and air control sections downstream of the abnormal compensation signal generating section are adversely affected. This means that a multiplex control system arrangement or a decentralized control system arrangement must be adopted in order to ensure the reliability of the control system, resulting inevitably in an expensive system.